fix(db): enforce IF EXISTS on DROP INDEX CONCURRENTLY for replay idempotency

Symmetric with the CREATE INDEX CONCURRENTLY rule: a post-COMMIT DROP INDEX
CONCURRENTLY replays from the top on failure, so without IF EXISTS it aborts
re-dropping an already-gone index.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: TheodoreSpeaks

scripts/check-migrations-safety.test.ts

@@ -165,6 +165,13 @@ DROP INDEX CONCURRENTLY IF EXISTS "stale_idx";`
     expect(lintSql(sql)).toEqual([])
   })
 
+  test('DROP INDEX CONCURRENTLY without IF EXISTS is not idempotent', () => {
+    const sql = `COMMIT;
+--> statement-breakpoint
+DROP INDEX CONCURRENTLY "stale_idx";`
+    expect(rules(sql)).toEqual(['error:concurrent-drop-index-not-idempotent'])
+  })
+
   test('DROP INDEX CONCURRENTLY without a preceding COMMIT errors', () => {
     expect(rules('DROP INDEX CONCURRENTLY IF EXISTS "stale_idx";')).toEqual([
       'error:concurrent-drop-index-no-commit',

scripts/check-migrations-safety.ts

@@ -294,6 +294,13 @@ function classify(sql: string, createdTables: Set<string>, sawCommit: boolean):
         message:
           'Plain DROP INDEX takes an ACCESS EXCLUSIVE lock on the table for the whole drop. Use DROP INDEX CONCURRENTLY after a COMMIT; breakpoint (see packages/db/scripts/migrate.ts).',
       })
+    } else if (!/\bIF EXISTS\b/i.test(s)) {
+      matches.push({
+        kind: 'error',
+        rule: 'concurrent-drop-index-not-idempotent',
+        message:
+          'DROP INDEX CONCURRENTLY must be IF EXISTS — a failed run replays from the top and would abort re-dropping an already-gone index.',
+      })
     } else if (!sawCommit) {
       matches.push({
         kind: 'error',